The invention relates to compressor control in general, and more particularly to compressor control operative on the long term while preventing the occurrence of a surge upon any intervening demand for a substantial load reduction.
It is known to adjust the flow of fluid in a compressor by controlling successively the inlet valve and the bypass valve of the compressor. To this effect, a master-controller has been associated with the compressor control system imposing a pressure setpoint and receiving a pressure feedback. As shown in U.S. Pat. No. 3,380,650, the inlet valve is controlled downward only to a minimum positioning level, in order to avoid a nonsteady situation which would cause a surge. This limit has been called the surge point. It is also known from this patent to recognize such minimum positioning of the inlet valve by sensing a minimum horse power from the motor driving the compressor. Such minimum positioning, however, is not maintained by the system. It has been proposed in the cross-referenced patent application, while controlling the bypass valve, to automatically maintain the inlet valve at its minimum position, while using the current of the motor driving the compressor as an indicator of any deviation from the assigned minimum inlet valve position. It is now proposed to reach such minimum inlet valve position upon a sharp demand for lower pressure without exceeding the minimum.
A surge is known to occur in a centrifugal compressor when the back pressure of the load becomes greater than the compressor pressure. It is known to prevent such occurrence by using a blow-off, or bypass valve, to vent the compressor when the flow falls below a preset minimum. See for instance U.S. Pat. No. 3,863,110.
It is recognized that for continuous operation a minimum flow rate should at all time be maintained in order for the compressor to be ready to supply the new demand following a fall of the demand. Such minimum flow rate could be exceeded should the master-controller receive an extreme and sharp demand for control of the pressure. The object of the present invention is to provide a reliable minimum flow rate despite an abrupt fall of the demand. Such an approach insures continuous operation of a compressor system under extreme load demands without any risk of a surge.